Julian, Dick, Anne, George and Timmy return to Kirrin Cottage for the Christmas holidays. As a result of being off school due to illnesses, the four children have a tutor, Mr. Roland, during the holidays in order to catch up with the work they missed. One day during the holidays, the children visit Kirrin Farmhouse, and the owners allow George to show her cousins all around the farmhouse, and show them some of the old secret cubby-holes in the farmhouse. George asks Mrs Sanders if she can show her cousins the cupboard with the false back on it upstairs, but Mrs Sanders tells her she is cleaning the room as she has some guests staying with her. When searching in a hole in the wall, Dick finds a piece of paper, with instructions on it written in Latin. The four take it back to Kirrin Cottage and ask Mr. Roland what the instructions on the paper say, as he reads Latin. He tells them that it is something about a room facing east with eight wooden panels in it, and the panel must lead to somewhere! That night, George goes downstairs to see Timmy, as he was made to sleep outside because Mr. Roland didn't like dogs. George goes into Uncle Quentin's study to get the front door key, and finds Mr. Roland standing in the study. He tells George he heard a noise and came to investigate, but George finds it very suspicious and is determined to investigate with the others.
Uncle Quentin - the famous scientist with a wonderful knowledge. He has a very hot temper, but is very caring towards his family. He is husband to Aunt Fanny, father to George and uncle to Julian, Dick and Anne
Aunt Fanny - a kind and loving mother to George and aunt to Julian, Dick and Anne
Georgina (George) - a hot-tempered but warm-hearted and loyal tomboy.
Julian - a strong boy, who can be arrogant and superior but is very protective of his cousin and younger siblings, especially Anne.
Dick - a clever, energetic boy with an excellent sense of humour.
Anne - the youngest of the five.
Timmy - George's brave dog.
Mr. Roland - the evil tutor, who plans to steal the secret formula along with his two companions.